A material for cooling electronic and photonic systems with high power density is desirable. High power systems tend to generate heat in very localized areas, causing the devices used in such systems to overheat. Diode lasers and all highly integrated electronics require efficient cooling. The key to efficient cooling is to bring the thermal conductor in the closest possible proximity to the devices and create a good thermal contact. Typically, the semiconducting devices are soldered directly onto the thermal conductor. However, the best thermal conductors disadvantageously tend to have a much higher coefficient of thermal expansion than semiconductor devices. Thermal conductors made of diamond tend to have a lower coefficient of thermal expansion than semiconductors. Mechanical stresses are induced during temperature cycling that will tend to overstress the semiconductor devices leading to potential failures when only copper or only diamond is used as a heat sink for semiconductor devices.
The extraordinary mechanical, thermal, and electronic properties of carbon nanotubes have attracted widespread interest. The carbon nanotubes exhibit a very low coefficient of thermal expansion, high strength, high elastic modulus, and uniquely high thermal conductivity along their longitudinal axis, but exhibit only average properties orthogonal to longitudinal axis, similar to those of graphite. Carbon nanotubes have a very low thermal expansion coefficient on the order of the coefficient of thermal expansion of diamond. Carbon nanotubes have been incorporated into polymeric materials and stretched to enhance thermal and electrical conductivity and to reduce the coefficient of thermal expansion of the polymers. However, polymers are unsuitable for use as thermal conductors and grounding heat sinks for semiconductor devices. Common carbon materials, such as graphite, exhibit poor thermal and electrical conductivity unsuitable for use as thermal conductors for semiconductor devices.
A substrate material for diode lasers, for example, can be a thermal conductor with a required coefficient of thermal expansion between 3.0 and 8.0 ppm/K. The exact value of this coefficient of thermal expansion depends upon which material the diode laser is made of. Carbon nanotubes have a coefficient of thermal expansion of about 0 ppm/° C. with thermal conductivity of K equal to 1000 W/mK to 1500 W/mK. Copper has a thermal conductivity of K=400 W/mK. Copper, however, expands at a rate of 16.6 ppm/K and produces significant stresses and subsequent device deterioration when soldered directly onto the device. Alternatively, diamond heat sinks have been used. Diamond heat sinks have a coefficient of thermal expansion of about 1.3 ppm/K, which is too small to fit with semiconductor devices. A bonding adhesion of any material for diamond also presents another significant problem when using diamond heat sinks. To solve the problem of heat sinking in diode laser systems, copper is used for the heat sink material as an alloy with tungsten. Forming such an alloy adjusts the coefficient of thermal expansion to about 8 ppm/K, but undesirably cuts the thermal conductivity of copper in half, from 400 W/mK to 200 W/mK. These and other disadvantages are solved or reduced using the invention.